Biochemical data support, and some models predict, that the role of calcium ions in vertebrate and invertebrate skeletal muscles is to regulate the population of actin sites for active interaction with myosin cross-bridges. They include no effect of Ca ions on the intrinsic rate constants which describe actomyosin ATPase and chemomechanical energy transduction. In contrast, some physiological data support, and some other models predict, that the role of Ca ions is two-fold. They not only regulate the population of active cross-bridges but modify some of the intrinsic rate constants which describe cross-bridge kinetics. We propose to apply the technique of sinusoidal analysis to skinned and intact skeletal muscle systems activated to varying degrees by electrical or chemical methods and by increasing Ca ion concentration respectively. By this technique we can isolate three apparent rate constants for cross-bridge cycling in the range of 1.5 to 800 sec-1. We can also measure the population of the cross-bridges in various states of the cycle. We propose to determine if Ca ions have an effect on rate constants as well as population factors, and if so, to document the effect, localize the action of Ca ions to a part of the cycle, and attempt to reconcile the biochemical and physiological results noted above.